--Phase 3 Vemurafenib Data to be Presented in Plenary Session
and to be Included in ASCO's Official Press Program--

BERKELEY, Calif.--(BUSINESS WIRE)--May 18, 2011 - Plexxikon
Inc., a member of the Daiichi Sankyo Group, today announced that
data related to vemurafenib and PLX3397, two promising oncology
drugs in its pipeline, will be presented in ten separate
presentations at the American Society of Clinical Oncology (ASCO)
2011 Annual Meeting taking place June 3 through June 7, 2011 in
Chicago, Illinois.

Importantly, final data from the Phase 3 study (BRIM3) of
vemurafenib in metastatic melanoma will be presented in the General
Oncology Plenary Session on Sunday, June 5th:

The abstracts will be available for review online on May
18th, 2011 at 6 pm ET, at
www.asco.org. Abstracts are submitted in early 2011, while
comprehensive clinical data to date will be delivered during the
presentations at the Annual Meeting itself.

Vemurafenib is an investigational, novel, oral small molecule
for treating melanoma harboring the oncogenic BRAF mutation.
Plexxikon utilized its structure-guided chemistry platform to
discover vemurafenib, and initiated clinical development in 2006.
Plexxikon and Roche signed a license and collaboration agreement in
2006 to co-develop vemurafenib. Under a 2005 agreement, a DNA-based
companion diagnostic to identify patients whose tumors carry the
BRAF mutation, the cobas 4800 BRAF V600 Mutation Test, is being
co-developed by Roche and Plexxikon in parallel with the
therapeutic development of vemurafenib. Roche has submitted
applications with the health authorities in the U.S. and Europe for
market approval for vemurafenib, and in the U.S. for the companion
diagnostic. The test will also be registered in Europe.

About PLX3397

PLX3397 is an orally available inhibitor that selectively
co-inhibits three key targets—FMS, Kit and
FIt3-ITD—allowing for down-modulation of a number of cell
types, including macrophages, microglia, osteoclasts and mast
cells, as well as selectively targeting the Flt3 mutation, a driver
in AML. Growth factors for these cells are elevated in significant
subsets of different human cancers, including glioblastoma, AML,
breast, colorectal, lung and prostate cancer. These growth factors
consequently activate the target cells, leading to a
microenvironment that supports tumor growth and enables metastases
to distant sites, particularly to bone. Additionally, PLX3397 has
the ability to penetrate the blood-brain barrier, indicating
potential efficacy in central nervous system primary or metastatic
tumors, as well as a range of other diseases. Plexxikon is
currently conducting a Phase 2 trial of PLX3397 in Hodgkin
lymphoma.

About Plexxikon

Plexxikon, a member of the Daiichi Sankyo Group, is a leader in
the structure-guided discovery and development of novel small
molecule pharmaceuticals to treat human disease. The company's lead
compound, vemurafenib (PLX4032), is in late-stage clinical trials
for the treatment of melanoma, and the subject of recent
applications for marketing approval in the U.S. and Europe.
PLX3397, the company's next oncology candidate, has advanced to
Phase 2 testing. The company is developing a portfolio of clinical
and preclinical stage compounds to address significant unmet
medical needs in oncology, as well as in several other therapeutic
indications. Plexxikon's proprietary Scaffold-Based Drug
Discovery™ platform integrates multiple state-of-the-art
technologies, including structural screening as a key component
that provides a significant competitive advantage over other drug
discovery approaches.

In April 2011, Daiichi Sankyo acquired Plexxikon. Plexxikon
continues research and development operations as an independent
unit of and member of the Daiichi Sankyo Group. Under Plexxikon's
U.S. co-promotion agreement with Genentech signed in 2010, Daiichi
Sankyo, Inc. will co-promote vemurafenib, subject to market
approval by FDA. For more information, please visit
www.plexxikon.com.